wing airflow - a different "view" via CFD 1

[amorrison]

Consider the "visuals" of the following:

1/ "Standard" wind tunnel wing "view" c/w streamlines. - At low AoA the air is leaving the trailing edge at an angle of -say- 15 degrees below HORIZONTAL

2/ A helicopter rotor "disk" rotating in the horizontal plane. - At low blade AoA the airflow is moving "only" VERTICALLY STRAIGHT DOWN

So why the difference?

In 1/ the air is moving over the stationary WING

In 2/ the wing is "cutting" through the stationary AIR

Why in "all" textbooks is option 1/ chosen? Maybe this was the "standard" and "only" option before CFD visuals.

Why not use CFD to promote the visuals of option 2 which in my opinion is the more "truthful" option since in the end lift is the reaction force to air being "forced" vertically downward by the "wing" shape.

 

[Sparweb]

Either model will give you the same information. Since wind tunnels were the way to get data in the golden days, it definitely follows that 1/ was the option of choice. Air leaving the trailing edge of any wing is imparted a vertical velocity, whether the wing is moving and the air is stationary, or vice-versa. The angle of downwash is a bit more meaningful in the wind tunnel as:

e = arctan(^{downwash velocity}/_{wind velocity})

When you consider the air to be still, and the aircraft to be moving, the equation loses meaning unless you re-name the wind velocity to be "flight velocity". Many textbooks do this interchangeably without much thought. Since CFD is still being corroborated with wind tunnel tests these days, new textbooks will probably continue coming out using the wind-tunnel view.

STF

 

[amorrison]

Some descriptive "DOWNFLOW" here by Boeing experts

http://www.amasci.com/wing/straub.txt

BUT still not a single diagram (CFD or other) showing DOWNFLOW posted in response yet (three months).